Induction Motor Tests Quiz

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Questions and Answers

What is the purpose of the blocked-rotor test?

To evaluate the motor's performance at rated current (correct)

To analyze the relationship between voltage and resistance

To measure the efficiency of the generator

To determine the input current under no load conditions

In the circle diagram construction, what does the angle $d$ represent?

The angle using the rated phase voltage and blocked rotor current

The angle between the load current and the stator copper loss

The angle between the current scale and the Y-axis (correct)

The angle between the output line and the torque line

What is meant by 'synchronous watts' in the context of the circle diagram?

The power supplied to the motor during idle operation

The power used to overcome internal resistance in the circuit

The component of power factoring in speed and torque (correct)

The maximum power the motor can generate without slip

How is the rotor copper loss related to the stator copper loss in the blocked-rotor test?

It is expressed as rotor copper loss/statotor copper loss = $r_2e T$ Signup and view all the answers

What does point P represent in the context of the torque line in the circle diagram?

The maximum torque point tangent to the semicircle Signup and view all the answers

To find the maximum torque and slip corresponding to maximum torque, which method is used?

Drawing a line parallel to the torque line as a tangent to the semicircle Signup and view all the answers

What total input parameters can be extrapolated from point P2 when working as a generator?

Generator output and input based on the power scale Signup and view all the answers

What type of current scale is suitable for the blocked-rotor test?

1 cm = 1 ampere Signup and view all the answers

What is the purpose of the no-load and blocked rotor tests on a 3-phase squirrel cage induction motor?

To calculate parameters of the equivalent circuit Signup and view all the answers

In the context of the circle diagram for a 3-phase induction motor, what does the diameter of the semi-circle represent?

The maximum value of current Signup and view all the answers

What happens to the current locus of a 3-phase induction motor as resistance increases from 0 to 1?

It becomes a semi-circle Signup and view all the answers

What parameters can be obtained from the circle diagram for a squirrel cage induction motor?

Line current, power factor, and slip at full load Signup and view all the answers

What does slip refer to in the context of an induction motor's operation?

The difference between synchronous speed and rotor speed Signup and view all the answers

During the no-load test, what is the primary objective once the steady state is reached?

To obtain and record no load readings Signup and view all the answers

What accessory is used to gradually apply the rated voltage during the no-load test of a squirrel cage motor?

Auto transformer starter Signup and view all the answers

What characteristic is plotted against output to obtain the torque-slip characteristics?

Current Signup and view all the answers

Study Notes

Objectives of the Tests

Draw the circle diagram and obtain torque-slip characteristics for slip ranging from 0 to 1.
Plot current, torque, power factor, efficiency, and slip against output characteristics for both stable and unstable regions.
Determine the line current, power factor, and slip at full load output using the circle diagram.
Predetermine maximum torque and corresponding slip from the circle diagram.
Obtain the rotational speed when the motor operates as a generator using the circle diagram.

Equipment Required

Motor: 3-phase squirrel cage induction motor, 5HP, 7.3A, 1440 RPM, 50 Hz, 400/440 V.
Ammeter: MI 0-5A, 0-10A; MC 0-5A.
Voltmeter: MI 0-600V, 0-150V; MC 0-15V.
Wattmeter: 150V, 10A (upf); 500V, 5A (ipf).
Other: 1002, 5A rheostat.

Principle of Testing

No-load and blocked rotor tests provide data to calculate induction motor equivalent circuit parameters.
Equivalent circuit behaves like a series R-L circuit with constant reactance and variable resistance.
Current locus resembles a semi-circle, with the diameter indicating maximum current.

Procedure

No-Load Test:
- Connect motor through an auto transformer starter.
- Gradually apply rated voltage to minimize starting current.
- Obtain steady-state no-load readings noted in a table.
Blocked-Rotor Test:
- Connect the motor via an auto transformer and physically block the rotor.
- Gradually increase voltage until rated current is reached.
- Record meter readings and measure winding resistance.

Data Processing

Draw the circle diagram using required data: ( I_o, I_{sc}, I_{2}' ) and se.
( I_{2}' ) (blocked rotor current at rated voltage) is calculated with ( I_{se} \times \text{Rated phase voltage} ).
Choose a suitable current scale (e.g., 1 cm = 1 ampere).
Power scale defined as ( 1 \text{ cm} = 3 \times V_{ph} \times \text{Current scale} ).

Circle Diagram Construction

Plot the current vector ( OA ) at angle ( d ) to Y-axis with length equivalent to current scale.
Draw vector ( OB ) at angle ( se ) such that length matches ( I_{2} ) to scale; join points A and B.
Construct a perpendicular bisector to line AB, meeting the horizontal line through A at point D; draw a semi-circle with radius DA.
Drop a vertical from B to line AD to establish point E, and select point F on BE for rotor and stator copper loss ratio.
Define output line AB and torque line AF; plot points ( L_{i} ) on the semi-circle and calculate respective values for torque, power loss, efficiency, and slip.
Analyze maximum torque by drawing a tangent to the torque line that contacts the circle; this gives maximum torque and corresponding slip.

Results

Successfully draw the circle diagram and plot performance characteristics based on the data obtained, providing detailed insights into the motor's behavior under different loads.

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Description

This quiz covers no-load and blocked rotor tests on 3-phase squirrel cage induction motors. Participants will draw circle diagrams and analyze torque-slip characteristics, exploring various factors like current, torque, power factor, and efficiency across stable and unstable regions.